By "a semiconductor diode operating as a detector" is meant in this application a semiconductor diode which is intended for and used as a radiation detector. The most common use is as a photodiode, that is, as a detector for optical radiation inside or outside the visible wavelength range. A diode of the intended kind operates under reverse bias. At the pn junction of the diode a depletion layer is then generated. Incident radiation generates electron-hole pairs in the depletion layer and thereby gives rise to a current through the diode, and the magnitude of this current is a measure of the intensity of the radiation.
Photodiodes are usually designed as vertical pin diodes, that is, with a relatively highly doped p-conducting layer at one surface of a semiconductor body, a relatively highly doped n-conducting layer at the opposite surface, and between these layers a weakly doped layer, in which the depletion layer is formed. The p-layer is provided with an anode contact and the n-layer with a cathode contact. Diodes of this kind will have a homogeneous depletion layer and thus good detection properties--the sensitivity is the same independently of where on the surface of the diode the radiation is caused to become incident. However, they must be designed with a relatively large area to attain the necessary sensitivity, and this results in a high self-capacitance and hence a low speed of operation.
Further, lateral photodiodes are known. In such, however, it has not been possible to attain the homogeneous depletion layer which the vertical diodes exhibit, and these diodes have therefore exhibited inferior detection properties.
To obtain good properties these diodes have been made of high-resistance and very high-quality (defect free) silicon, typically with a resistivity of 500-1000 ohmcm.
It is further known to design detector diodes with integrated amplifiers. The diode and the amplifier are then produced in an epitaxial silicon layer arranged on a highly doped substrate. One of the contact regions of the diode is then produced at the same surface as the amplifier, and the other contact region of the diode consists of the substrate layer. Thus, the diode is made as a vertical diode, and its depletion layer is formed in the epitaxial layer. Such a layer is well suited for amplifier circuits, for example, of CMOS type, but because of the lower silicon quality it provides an inferior function of the detector diode.